During the operation of a fluidized bed catalytic cracking unit (hereinafter FCC), the catalyst accumulates coke. The degree of catalyst coking is related to the process conditions in the reactor riser with more severe cracking conditions increasing the degree of coke deposition. Cracking a higher boiling point feedstock or raising the reactor riser temperature increases cracking severity and consequently increases coke production. Coke blocks access to the pores of the catalyst and must be removed to restore catalytic activity. Removal of coke in the regenerator is exothermic and the heat generated is directly proportional to the amount of coke burned off the catalyst.
Regeneration of the spent catalyst in many applications produces more heat than is required to vaporize and crack the hydrocarbon feedstream entering the reactor riser. Excessively high regenerated catalyst temperatures in the reactor riser are undesirable and decrease gasoline and distillate yields while increasing the production of coke and C.sub.4 and lighter hydrocarbons. Therefore, it is advantageous to cool the regenerated catalyst to within an optimum temperature range before it enters the reactor riser.
This invention relates to integrating the dehydrogenation and aromatization of a lower C.sub.3 -C.sub.5 alkane, preferably propane, with the operation of an FCC unit. The dehydrogenation and aromatization of the alkane feedstream is carried out in a fluidized catalyst bed which is divided by a gradual change in catalyst concentration into two reaction zones. The large-pore cracking catalyst is concentrated in the lower section of the reactor and the medium-pore additive catalyst is concentrated in the upper section of the reactor. Specifically, thermal dehydrogenation occurs in the lower section of the reactor in the presence of the large-pore acid zeolite cracking catalyst while the aromatization occurs in the upper section of the reactor in the presence of the medium-pore acid zeolite additive catalyst.
The thermal dehydrogenation of normally liquid hydrocarbons at a temperature in the range from 538.degree. C. to 750.degree. C. (1000.degree. to 1382.degree. F.) by pyrolysis in the presence of steam, is disclosed in U.S. Pat. Nos. 3,835,029 and 4,172,816, inter alia, but there is no suggestion that such a reaction may be used as the basis for a direct heat exchange, to cool regenerated catalyst in an external catalyst cooler for an FCC unit.
FCC regenerators with catalyst coolers are disclosed in U.S. Pat. Nos. 2,377,935; 2,386,491; 2,662,050; 2,492,948; and 4,374,750 inter alia. These previous designs remove heat by indirect heat exchange, typically a shell and tube exchanger. None removes heat by direct heat exchange, for example, by continuously diluting hot regenerated catalyst with cold catalyst, or by blowing a cold gas through the hot catalyst; in particular, none removes heat by functioning as a reactor which supplies heat to an endothermic reaction.
The cooling of hot regenerated catalyst via an endothermic reaction, specifically the catalytic dehydrogenation of butane, was disclosed in U.S. Pat. No. 2,397,352 to Hemminger. Though unrelated to operation of an FCC unit, regeneration of the catalyst was required before it was returned to the dehydrogenation reactor. A catalyst heating chamber was provided for supplying heat to the reaction to compensate for that lost in dehydrogenation, and to preheat the butane feedstock.